Wood-Steel Composite Beam?

Q.We’re considering through-bolting LVL lumber to both sides of a steel I-beam. We’d install the resulting composite beam flush at midspan in an existing floor system, with the LVL providing convenient nailing for joist hangers. Can we assume that the combined load-bearing values for LVL and steel will produce a stronger, stiffer beam than either material on its own?

A.John Bologna, P.E., of Coastal Engineering, in Orleans, Mass., responds: It’s not that simple, because the two materials have widely different mechanical properties. Simply stated, the stiffer member will always carry the greater percentage of the load. Given both the high modular value of steel over wood and the engineering effort required to analyze and detail a properly connected composite section, I typically wouldn’t attribute any value to the wood section and would count on the steel alone to do the work.

To determine the share of load carried by each member, you have to take the relative stiffness of each into account. “Stiffness” is defined as the ratio of a given material’s properties in the form of EI/L, where “E” is the elastic modulus, “I” is the member’s moment of inertia, and “L” is the span length. To carry a load in unison, sufficient connectors would be required to transfer static shear between each member. Then, you’d have to calculate the resulting composite section using a modular ratio to convert a wood section to an equivalent steel section (or vice versa). The modular ratio is the relationship between the elastic moduli of both materials and has a nondimensional value on the order of 14.5 for steel over wood. In other words, steel is about 14.5 times stronger than wood.

Depending upon the specific application, you may need to fabricate an all-steel composite section to achieve the desired properties. But in a flush framing situation, I would suggest considering the LVL as merely a nailer for hanging the floor joists.